In the fabrication of semiconductor integrated circuits, metal conductor lines are used to interconnect the multiple components in device circuits on a semiconductor wafer. A general process used in the deposition of metal conductor line patterns on semiconductor wafers includes deposition of a conducting layer on the silicon wafer substrate; formation of a photoresist or other mask such as titanium oxide or silicon oxide, in the form of the desired metal conductor line pattern, using standard lithographic techniques; subjecting the wafer substrate to a dry etching process to remove the conducting layer from the areas not covered by the mask, thereby leaving the metal layer in the form of the masked conductor line pattern; and removing the mask layer typically using reactive plasma and chlorine gas, thereby exposing the top surface of the metal conductor lines. Typically, multiple alternating layers of electrically conductive and insulative materials are sequentially deposited on the wafer substrate, and conductive layers at different levels on the wafer may be electrically connected to each other by etching vias, or openings, in the insulative layers and filling the vias using aluminum, tungsten or other metal to establish electrical connection between the conductive layers.
Deposition of conductive layers on the wafer substrate can be carried out using any of a variety of techniques. These include oxidation, LPCVD (low-pressure chemical vapor deposition), APCVD (atmospheric-pressure chemical vapor deposition), and PECVD (plasma-enhanced chemical vapor deposition). In general, chemical vapor deposition involves reacting vapor-phase chemicals that contain the required deposition constituents with each other to form a nonvolatile film on the wafer substrate. Chemical vapor deposition is the most widely-used method of depositing films on wafer substrates in the fabrication of integrated circuits on the substrates.
Due to the ever-decreasing size of semiconductor components and the ever-increasing density of integrated circuits on a wafer, the complexity of interconnecting the components in the circuits requires that the fabrication processes used to define the metal conductor line interconnect patterns be subjected to precise dimensional control. Advances in lithography and masking techniques and dry etching processes, such as RIE (Reactive Ion Etching) and other plasma etching processes, allow production of conducting patterns with widths and spacings in the submicron range. Electrodeposition or electroplating of metals on wafer substrates has recently been identified as a promising technique for depositing conductive layers on the substrates in the manufacture of integrated circuits and flat panel displays. Such electrodeposition processes have been used to achieve deposition of the copper or other metal layer with a smooth, level or uniform top surface. Consequently, much effort is currently focused on the design of electroplating hardware and chemistry to achieve high-quality films or layers which are uniform across the entire surface of the substrates and which are capable of filling or conforming to very small device features. Copper has been found to be particularly advantageous as an electroplating metal.
Electroplated copper provides several advantages over electroplated aluminum when used in integrated circuit (IC) applications. Copper is less electrically resistive than aluminum and is thus capable of higher frequencies of operation. Furthermore, copper is more resistant to electromigration (EM) than is aluminum. This provides an overall enhancement in the reliability of semiconductor devices because circuits which have higher current densities and/or lower resistance to EM have a tendency to develop voids or open circuits in their metallic interconnects. These voids or open circuits may cause device failure or burn-in.
FIG. 1 schematically illustrates a typical standard or conventional electroplating system 10 for depositing copper onto a semiconductor wafer 18. The electroplating system 10 includes a standard electroplating cell having an adjustable current source 12, a bath container 14, a copper anode 16 and a cathode 18, which cathode 18 is the semiconductor wafer that is to be electroplated with copper. The anode 16 and semiconductor wafer/cathode 18 are connected to the current source 12 by means of suitable wiring 38. The bath container 14 holds a bath 20 typically of acid copper sulfate solution which may include an additive for filling of submicron features and leveling the surface of the copper electroplated on the wafer 18.
As illustrated in FIGS. 1 and 2, the electroplating system 10 typically further includes a pair of bypass filter conduits 24 which extend through the anode 16 and open to the upper, oxidizing surface 22 of the anode 16 through respective sludge openings 26 at opposite ends of the anode 16. The bypass filter conduits 24 connect to a bypass pump/filter 30 located outside the bath container 14, and the bypass pump/filter 30 is further connected to an electrolyte holding tank 34 through a tank inlet line 32. The electrolyte holding tank 34 is, in turn, connected to the bath container 14 through a tank outlet line 36.
In operation of the electroplating system 10, the current source 12 applies a selected voltage potential typically at room temperature between the anode 16 and the cathode/wafer 18. This potential creates a magnetic field around the anode 16 and the cathode/wafer 18, which magnetic field affects the distribution of the copper ions in the bath 20. In a typical copper electroplating application, a voltage potential of about 2 volts may be applied for about 2 minutes, and a current of about 4.5 amps flows between the anode 16 and the cathode/wafer 18. Consequently, copper is oxidized typically at the oxidizing surface 22 of the anode 16 as electrons from the copper anode 16 and reduce the ionic copper in the copper sulfate solution bath 20 to form a copper electroplate (not illustrated) at the interface between the cathode/wafer 18 and the copper sulfate bath 20.
The copper oxidation reaction which takes place at the oxidizing surface 22 of the anode 16 is illustrated by the following reaction formula (1):Cu→Cu +++2e−  (1)
The oxidized copper cation reaction product forms ionic copper sulfate in solution with the sulfate anion in the bath 20:Cu+++SO4−−→Cu++SO4−−  (2)
At the cathode/wafer 18, the electrons harvested from the anode 16 flowed through the wiring 38 reduce copper cations in solution in the copper sulfate bath 20 to electroplate the reduced copper onto the cathode/wafer 18:Cu+++2e−→Cu  (3)
As the anode 16 is consumed during the electroplating process, small quantities of solid copper sulfate or “anode fines” tend to precipitate at the interface between the copper sulfate bath 20 and the oxidizing surface 22 of the anode 16 to form a copper precipitate or sludge 28 on the oxidizing surface 22, as illustrated in FIG. 2.
Various problems can be caused by the sludge 28 on the anode 16. For example, the sludge 28 may cause a voltage drop in the electroplating cell because oxidixed copper ions must migrate through the sludge in order to reach the bath solution 20. The sludge 28 may also affect deposit uniformity of the copper on the wafer 18. Additionally, the anode sludge 28 can be the source of potential wafer-contaminating particles which may contaminate the copper plated onto the wafer 18.
Copper sludge 28 can normally be effectively removed from the oxidizing surface 22 by operation of the bypass pump/filter 30, wherein the bath solution 20 is continually drawn through the sludge openings 26 of the anode 16 and to the electrolyte holding tank 34 through the bypass filter conduits 24, bypass pump/filter 30 and tank inlet line 32, respectively. The bypass pump/filter 30 removes the particulate precipitate/sludge 28 from the bath solution 20 before entry of the bath solution 20 into the electrolyte holding tank 34. The filtered bath solution 20 is typically distributed from the electrolyte holding tank 34 back into the bath container 14 through a tank outlet line 36 to replenish the supply of the bath solution 20 in the bath container 14.
As further illustrated in FIG. 2, in its original condition the anode 16 is typically rectangular in cross-section and has a uniformly flat oxidizing surface 22. During prolonged use of the anode 16 in the electroplating system 10, however, copper from the oxidizing surface 22 of the anode 16 is oxidized and enters the copper sulfate solution in the bath 20, as indicated by reactions (1) and (2), respectively, above. Consequently, as the copper is gradually removed from the oxidizing surface 22 of the anode 16, the oxidizing surface 22 gradually assumes a concave profile, as illustrated in FIG. 3. The sludge 28 tends to accumulate on the concave oxidizing surface 22, as illustrated in FIG. 3, and is more difficult to remove from the concave oxidizing surface 22 than from the relatively flat oxidizing surface 22. Accordingly, small particles from the sludge 28 may break off and enter the bath 20 and potentially contaminate the wafer 18 during the electroplating process. Consequently, the concave anodes 16 must be frequently replaced during periods of frequent usage of the electroplating system 10.
Accordingly, an electroplating anode is needed which is more resistant to concave profiling during prolonged wafer electroplating and which extends the lifetime of the anode in the electroplating system.